This invention relates to apparatus for urging a workpiece against the surface of a chuck of a lathe or the like to securely hold the workpiece at a correct position.
When mounting a workpiece on a chuck of a machine tool with a hand of a robot machine, there are such problems as an inclination of the axis of the workpiece with respect to the axis of the chuck (hereinafter termed a deflection angle) and displacement of the axis of the workpiece from that of the chuck (hereinafter termed an eccentricity). The operation of the chuck for holding the workpiece will now be described with reference to FIGS. 1A through 1D. Referring first to FIG. 1A, when a workpiece 1 is grasped by a hand of a robot machine, for example, and then grasped by a chuck, a deflection angle .delta. is often occurs between the workpiece 1 and the chuck 2. In such a case, a press device is operated as shown in FIG. 1B to urge its press plate 3 against the rear surface of the workpiece so as to eliminate the inclination of the workpiece 1 by the force F applied to the press plate. As the chuck 2 clamps further, the eccentricity e present in the state, as shown in FIG. 1B, is eliminated so that the workpiece 1 can be held by the chuck 2 without any deflection angle and eccentricity.
As will be seen from the comparison of FIGS. 1A and 1B, the control of the workpiece effected by the press plate utilizes a slip between the workpiece 1 and the press plate 3, and in the absence of such a slip, the workpiece 1 would be held by the chuck 2 without eliminating the deflection angle as shown in FIG. 1D. FIG. 2 shows the detail between the slip and the dimension of the workpiece 1. It will be appreciated from the figure that where the length L of the workpiece 1 is larger than a predetermined value, a slip represented by L.multidot..DELTA..theta. appears whereas no slip occurs unless the friction coefficient between the workpiece and the press plate 3 in less than l/L. These facts were confirmed by our experiment.
As described above, in certain cases, depending upon the dimension of the workpiece, it is impossible to eliminate the deflection angle by the slippage between the workpiece and the pressing plate. Accordingly, it has been desired to develop an improved apparatus capable of urging the workpiece without resorting to slippage.